2 Answers
2

As I said in a comment above, setting query_var => false for the custom taxonomy is not a solution because this will make term pages in frontend return a 404 error. Renounce to term pages in the frontend to make a simple form to work is far away to be a solution.

Finally I've solved the problem. Instead of set the name in the select element to 'my_tax[]', I've set it to something like 'tax_input['my_tax'][]. Don't ask me why Wordpress needs this, but it fix the problem.

That is a poor workaround. Setting query_var to false will made permalinks to terms pages return 404 error. Other solutions is needed if the terms pages are needed in the frontend, as it is my case.
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cybmetaSep 4 '13 at 23:34